In a hydraulically operated braking system for wheeled motor vehicles, wherein front and rear wheels are braked simultaneously, if the rear wheels are locked first, the vehicle is likely to show rear end skid or "yaw." In fact, such locking is far more dangerous than if the the front wheels are locked first. In view of the fact that upon braking during forward cruising, the effective weight transfer of the vehicle occurs reducing the load on the rear wheels thereby causing the rear wheels to lock easier than the front wheels, a hydraulic pressure control valve is usually provided in a rear wheel braking hydraulic line for limiting the rise of hydraulic pressure exerted in the hydraulic line leading to the rear wheel brake cylinders, with respect to the rise of pressure in the line leading to the front wheel brake cylinders.
As a hydraulic pressure control valve of the kind referred to, a proportioning valve (which will be referred to as "P-valve" hereinafter) has been proposed. In the P-valve, the pressure value when beginning to limit the rise of the rear wheel braking hydraulic pressure, that is, the critical hydraulic pressure, is constant thereby to make the front and rear wheels brake pressure distribution characteristic constant.
As is known, however, an ideal distribution characteristic for locking (not braking) the front and rear wheels simultaneously is one that varies with the change in the vehicle weight. More specifically, the ideal characteristic is such that the critical hydraulic pressure becomes higher as the vehicle weight increases. Thus, the P-valve is inapplicable to trucks and the like the weight of which varies greatly depending on whether they are empty or loaded.
In view of the above, there has been proposed a hydraulic pressure control valve assembly which comprises a combination of the P-valve and a deceleration sensing valve (which will be referred to as G-valve hereinafter), which detects any deceleration of a vehicle and operates to raise the critical hydraulic pressure in proportion to the increase of the vehicle weight.
With a hydraulic pressure control valve assembly of such a structure, however, when the load is maximum or when the front wheel braking system fails to operate, sufficient critical hydraulic pressure cannot be produced, so that the braking force for the vehicle becomes insufficient. In order to solve this problem, some expedients are put into practical use; for example, increasing the setting load of the spring which acts on a plunger member of the P-valve, increasing the installation slanting angle of the hydraulic pressure control valve with respect to the horizontal, or increasing the spring constant of the said spring. Two former ways, though of achieving the object, result in the critical hydraulic pressure at the time when vehicle is empty or lightly loaded becoming too high causing the front and rear wheels brake pressure distribution characteristic to be outside the ideal range, thereby causing the rear wheels to lock at such time. The third way results in not only the critical hydraulic pressure becoming too high around the time when the vehicle is half loaded resulting in the front and rear wheels brake pressure distribution characteristic being outside the ideal range, thereby causing the rear wheels to lock at such time, but also that the stress applied to the plunger member of the P-valve considerably increases thereby causing a durability problem and enlargement of the spring, and also of the hydraulic pressure control valve assembly, itself.